1. Field of the Invention
This invention relates to a digital filter wherein a plurality of delay elements are used to obtain an output signal composed of a plurality of delay signals being different in delay time and different in signal level from one another, and more particularly, to improvements in the digital filter capable of decreasing the number of multipliers utilized in the digital filter and of reducing the costs.
2. Description of the Prior Art
There has heretofore been known that a transfer function of a predetermined frequency response can be realized by an impulse response of a certain type. Furthermore, there have been known various theories wherein the impulse response of the transfer function forming the predetermined frequency response as described above is sought for utilizing it in electronic components and the like.
In order to obtain the predetermined frequency response in accordance with the above-described theories, a digital filter is obtained in such a manner that a plurality of delay elements are used to obtain an output signal composed of a plurality of delay signals which are different in delay time from one another and which are different in signal level from one another, so that the filter having the corresponding impulse response can be realized.
Digital filters using the plurality of delay elements as described above, are known as finite impulse response filters (hereinafter referred to as a "FIR filter") and infinite impulse response filters (hereinafter referred to as an "IIR filter").
Incidentally, differences and similarities between the FIR filter and the IIR filter will be described later with reference to FIGS. 17 and 18.
FIG. 13 is a block diagram showing the conventional FIR filter.
In this FIG. 13, delay elements D.sub.1 -D.sub.n totalling to n are serially connected to one another, and taps totalling to n+1 are provided at respective connecting portions in order to obtain delay signals having various delay times. Furthermore, a filter input signal inputted through an input terminal IN is inputted into input terminals of the delay element D.sub.1 and a multiplier M.sub.0. Further, multipliers M.sub.0 -M.sub.n totalling to n+1, capable of receiving delay signals different in delay time from one another and of obtaining delay output signals each having an optional signal level are provided at the taps, respectively. The delay output signals from the multipliers totalling to n+1 are added together by an adder A and outputted into an output terminal OUT as a filter output signal.
In the above-described FIR filter, delay signals are obtained from the taps on the output sides of the delay elements, the said taps being located at predetermined positions from the input terminal IN (i.e., predetermined delay times), and these predetermined delay signals are turned into delay output signals each having a desired signal, level, by the multipliers disposed at the respective taps, whereby all of these delay output signals are added together by the adder, thus obtaining a final filter output. With this arrangement, in the above-described FIR filter, the filter output signal having a desired impulse response can be obtained for the filter input signal inputted into the input terminal IN.
In FIG. 13, the filter input signal inputted through the input terminal IN is data continued at intervals of a predetermined sampling time Ts. Furthermore, the FIR filter in FIG. 13 performs the digital process in accordance with a clock having a cycle of the above-described sampling time Ts. The data being filtered (i.e., the input signal inputted through the input terminal IN) is inputted into the delay element D.sub.1 and successively shifted to the delay elements D.sub.2 . . . D.sub.n with every clock.
Incidentally, when the FIR filter shown in this FIG. 13 is used for the image processing by the NTSC (National Television System Committee) method, 4 fsc=14.3 MHz is frequently used as the frequency of the clock, i.e., sampling frequency in accordance with a subcarrier frequency fsc=3.58 MHz.
Incidentally, these delay elements D.sub.1 -D.sub.n are registers for storing words of a predetermined bit number, e.g., 8 bit words, and perform fixed delaying by one clock sampling time, Ts.
Data of delay times in accordance with the number of the delay elements D.sub.1 -D.sub.n which have passed can be obtained from the taps disposed between the delay elements D.sub.1 -D.sub.n. Namely, a signal (data) delayed by a time (n.times.Ts) can be obtained from the tap on the output side of the number n delay element D.sub.n.
These taps are connected with the multipliers M.sub.0 -M.sub.n corresponding thereto, and coefficients a.sub.0 -a.sub.n set at every multiplier M.sub.0 -M.sub.n are applied to the signals (data) from the taps.
Incidentally, these coefficients a.sub.0 -a.sub.n are coefficients for about 8-10 bits. Furthermore, as the multipliers M.sub.0 -M.sub.n, parallel multipliers are normally used for high speed processing. For example, when the clock frequency of the above-mentioned clock is set at 14.3 MHz, the clock cycle (sampling time Ts) becomes (1/14.3 MHz=70 ns), so that the operational speeds of the multipliers M.sub.0 -M.sub.n must be faster than 70 ns.
Incidentally, the above-described parallel multipliers perform the operations in a substantially parallel manner to one another about the figures of multipliers and the figures of multiplicands, whereby many logical gates are required, and, in the parallel multipliers of 8.times.8 or 8.times.10 bit class, about 1000 gates are required.
In FIG. 13, all of the outputs from the multipliers M.sub.o -M.sub.n are added together by the adder A, and the result is outputted from the output terminal OUT. Additionally, the results of multiplying outputted from the multipliers M.sub.o -M.sub.n are data of 16-18 bits, the adder A can perform adding of 16-18 bits, and the result can be outputted from the output terminal OUT.
Incidentally, the operation performed in the FIR filter shown in FIG. 13 can be expressed as in the following equation where the number input at the intervals of sampling time Ts is X.sub.k, an output thereof is Y.sub.k and coefficients applied at the multipliers M.sub.o -M.sub.n are a.sub.0 -a.sub.n, respectively. ##EQU1##
Incidentally, the above-described operations are the operations called convolution, and, the operations of this type can give some characteristic of frequency to the digital filter. Furthermore, this characteristics of frequency can be determined by way of a coefficient a.sub.i.
Furthermore, there have heretofore been disclosed techniques of removing a ghost signal from a signal received by a television to improve a ghost screen by use of various filters.
FIG. 14 is an explanatory view showing the ghost screen wherein the ghost signal is superimposed on the signal received together with the main signal.
In FIG. 14, an image I.sub.0 is a real image by the main signal, and an image I.sub.1 is a ghost image created by the ghost signal which has been superimposed on the main signal in the signals received.
A value t.sub.a of a shift between the real image and the ghost image is determined by a lag time or a lead time of the ghost signal which has been superimposed on the original signal. The ghost shifted to the right like the image I.sub.1 relative to the image I.sub.0 in FIG. 14 is called a post-ghost. On the other hand, the ghost shifted to the left in the screen is called a pre-ghost. This pre-ghost appears when the ghost signal is more advanced in the electric wave propagation than the original signal. However, out of the electric waves propagated by the delay times different from one another, the most intensive signal is the main signal, so that the above-described pre-ghost may appear.
Incidentally, FIG. 14 presupposes the NTSC method, whereby the horizontal scan from left to right is successively performed from to top to bottom. Furthermore, a horizontal scan cycle T.sub.h is 63.5 microseconds, about 80% of the horizontal scan is displayed in the screen, and the right and left portions of the horizontal scan, which are not displayed on the screen, are called horizontal blankings.
FIG. 15 is a view of electric wave propagation for explaining the process of generation of the ghost.
In FIG. 15, a direct wave B of the broadcast wave radiated from a broadcast station 20 reaches an antenna 24 by the shortest distance. On the other hand, parts of the broadcast wave radiated from the broadcast station 20 reach the receiving antenna 24 as reflected waves C and D reflected at concrete and steel buildings 22a and 22b. Since these reflected waves C and D are propagated for distances longer than the distance of propagation of the direct wave B, the propagation times thereof become longer then that of the direct wave B. Furthermore, since the surfaces reflecting the broadcast wave of the concrete and steel buildings 22a and 22b have some spaces, the propagation times of the reflected waves C and D have spreads, respectively, whereby the reflected waves C and D are turned into composite signals each including a multiplicity of reflected waves having propagation times close to one another. Accordingly, the received signal in the receiving antenna 24, becomes a signal which has been superimposed on not only the main signal but also the ghost signal having the delayed time.
FIG. 16 shows the wave forms of the original signal and the received signal in which the ghost signal is superimposed on the main signal corresponding to the original signal.
In FIG. 16, the original signal x(t) is displayed by a square wave having a height 1. Furthermore, the received signal y(t) in FIG. 16 superimposed thereon with the square wave g.sub.0 having the height 1 by the direct wave in general and square waves g.sub.1 -g.sub.5 of the ghost signals is produced by a plurality of reflected waves.
Furthermore, the square waves g.sub.1, g.sub.2, g.sub.3, g.sub.4 and g.sub.5 of the ghost signal produced by the reflected waves which have been superimposed thereon are delayed relative to a square wave g.sub.0 by the original signal produced by the direct wave by the delayed times .DELTA.t.sub.1, .DELTA.t.sub.2, .DELTA.t.sub.3, .DELTA.t.sub.4 and .DELTA.t.sub.5. Furthermore, signal levels of the square waves g.sub.1, g.sub.2, g.sub.3, g.sub.4 and g.sub.5 of the ghost signal are a.sub.1, a.sub.2, a.sub.3, a.sub.4 and a.sub.5, respectively.
The received signal y(t) shown in FIG. 16 can be expressed in the following equation. ##EQU2##
Modifying equation (2), x(t) can be sought in the following equation. ##EQU3##
Namely, the ghost signal which has been superimposed on the received signal y(t) can be removed by the operations of equation (3).
Furthermore, when the operations of equation (3) is described on the assumption of the digital process, i.e., described on the assumption of the debunching time system, the following equation can be established. ##EQU4##
There has heretofore been practiced by use of the above-described FIR filter to perform the operations according to the aforesaid equation (3) or (4) for effectively removing the ghost signal from the received signal.
This FIR filter can be realized by the digital filter. In recent years, with the decrease in the costs of digital filters, various ghost cancelers have been developed by the FIR filters formed of the digital filters.
FIG. 17 is a block diagram showing a first example of the ghost canceler using the digital filter.
In FIG. 17, the operations shown in the aforesaid equation (4) is performed. Namely, in FIG. 17, x(t) and y(t) correspond to those in equation (3). Reference numeral 12a designates the FIR filter as shown in FIG. 18. Furthermore, the digital filter shown in FIG. 17 has a feedback loop to the FIR filter 12a, thus presenting an IIR filter as a whole.
FIG. 19 is a block diagram showing a second example of the ghost canceler using the digital filter.
In FIG. 19, an input signal (received signal) inputted through the input terminal IN passes through an FIR filter comprising a digital filter constituted by delay elements of 64 taps, and is inputted into one of two input terminals of the adder A. An output from this adder A is inputted into an FIR filter 12b comprising a digital filter constituted by the delay elements of 576 taps, and an output from FIR filter 12b is inputted in the other of the two input terminals of the aforesaid adder A. Namely, FIR filter 12b and the adder A constitute an IIR filter, and an output from adder A is connected to an output terminal OUT of the ghost canceler as well.
The second example of the ghost canceler particularly has an FIR filter 10 used as an equalizing portion as compared with the aforesaid first example. FIR filter 10 is used for correcting the distortions in the wave forms in a transmission system from a receiving antenna to a television and for removing a ghost close to the main signal within the range of about plus or minus 2 microsecond.
Furthermore, the IIR filter constituted by the adder A and the FIR filter 12b constitutes a ghost cancelling portion in which the operations of the aforesaid equation (3) or (4) are performed, i.e., a plurality of delay signals being different in delay time from one another with the said delay signals made different in signal level from one another are added to thereby remove the ghost signal.
FIG. 20 is a block diagram showing a third example of the ghost canceler using the digital filter.
In FIG. 20, the FIR filter 10 is identical with the FIR filter having the same reference numeral in FIG. 19 and similarly constitutes an equalizing portion.
In FIG. 20, in the ghost canceling portion, 10 to 16 sets of signal delaying parts constituted by variable delayers 14 and FIR filter 12c having delay elements of 7-16 taps are arranged in parallel to one another, and outputs from the respective signal delaying parts, i.e., outputs of the respective variable delayers 14 are added together by the adder A.
The third example of the ghost canceler shown in FIG. 20 is constructed so as to decrease the total number of taps of the FIR filter on the basis of the fact that the number of the multipliers M.sub.o -M.sub.n, as shown in FIG. 18, having the value "0" is large in the first example of the ghost canceler.
In FIG. 20, in the variable delayer 14, as shown in FIG. 21, fixed delay elements DF.sub.1 -DF.sub.n are serially connected to one another. Furthermore, in variable delayer 14, the output terminal OUT is connected to a tap selected between the fixed delay elements DF.sub.1 -DF.sub.n, or switched from one tap to another so as to set the delay time.
In the third example of the ghost canceler, as shown in FIG. 22, the removal of ghost signals g.sub.11 -g.sub.13 which have been superimposed on a main signal g.sub.10 is performed by three variable delayers 14 and three FIR filters 12c.
Namely, as designated by reference numeral F1 in FIG. 22, a ghost signal g.sub.11 having a delay time .DELTA.t.sub.11 can be removed by the variable delayer VD1 and an FIR filter FIR 1. As denoted by reference numeral F2, a ghost signal g.sub.12 having a delay time .DELTA.t.sub.12 can be removed by the variable delayer VD2 and an FIR filter FIR 2. As indicated by reference numeral F3, a ghost signal g.sub.13 having a delay time .DELTA.t.sub.13 can be removed by the variable delayer VD3 and an FIR filter FIR 3. Namely, these ghost signals g.sub.11 -g.sub.13 can be removed by the number of taps of the FIR filters totalling to about (7.times.3 - - - 16.times.3=48).
Incidentally, theoretically, one tap is sufficient for a removal of one ghost. However, practically, the ghost has the spread, so that 7-16 taps (fixed number) are allocated to one ghost.
As described above, according to the third example of the ghost canceler, the total number of taps of the FIR filters, which is relatively small, can remove the ghost signal, whereby the total number of multipliers used can be reduced, thus reducing the costs.
However, the first and second examples of the aforesaid ghost canceler are characterized in that the plurality of delay signals having the delay times different from one another are formed into a composite output signal with the delay times and the signal levels being selected widely, and the ghost signals having various delay times and various signal levels can be effectively removed, while these examples have such a problem that the multi-tap FIR filter must be used.
The above-described multi-tap FIR filter requires a multiplicity of multipliers, thus presenting the problem of increasing the cost of the ghost canceler as a whole.
On the other hand, the third example of the ghost canceler as shown in FIG. 20 offers the advantage that the number of taps used in FIR filter may be small, thus reducing the cost of the ghost canceler as a whole. However, since the number of the FIR filters to be used and the number of taps of each of the FIR filters are limited, the number and spreads of ghost to be removed are limited, and, such problems are presented that, when a multiplicity of ghost signals having the delay times and the signal levels, which are different from one another are superimposed on the received signals, all of the ghosts cannot be removed, or the ghost signal having a large spread cannot be fully removed. Furthermore, such a problem is presented that 7-16 taps are uniformly allocated to a narrow ghost requiring only a small number of taps.